Is there a way to forbid using the "\" command in MATLAB Grader submissions?

When using MATLAB Grader to automatically evaluate submitted functions, I'd like to forbid my students using Matlab's built-in linear system solver (command "\", among other). To achieve this, I tried the assessment test:

assessFunctionAbsence({'\','mldivide'},'FileName','yoursolver.m');

where yoursolver.m is the submitted function. While the use of mldivide(A,b) is correctly identified, the use of "\" (its shorthand equivalent) is not, yoursolver.m containing "\" command passes this test. Any ideas how to identify the use of "\" and forbid its use? Thanks!

[Jan]

\ does call mldivide if the inputs are matrices. Can you add an overloaded function before evaluating the submitted functions?

function varargout = mldivide(varargin)
error('Function not allowed.')
end

I might be old-fashioned, but I tell my students not to use the \ operator and inspected the code personally. Of course this is more work, but an automatic grading is too easy to be tricked at all.

[Bruno Luong]

You also should also forbid "/" since A\b is (b'/A')', and also a bunch of functions related to linear solver.

[Ales Janka]

Hello Jan,

there is a way to upload M-files (fiield Files Referenced) which could include data and functions usable within the submitted function. However, when I upload the overloaded mldivide.m as you suggested, it is not used, even if I explicitely call mldivide(A,b) within the submitted function (question of priorities?). No effect for "\" usage either. Only a warning appears:

[Warning: Function mldivide has the same name as a MATLAB builtin. 
    We suggest you rename the function to avoid a potential 
    name conflict.] 
[> In matlab.internal.getCode (line 22) 
    In mtree (line 78) 
    In FileReadHelper/generateMtree 
    In FileReadHelper/generateFileMetadata 
    In FunctionAbsenceConstraint/getNotAbsentFunctions 
    In FunctionAbsenceConstraint/satisfiedBy 
    In assessFunctionAbsence 
    In ScoringEngineTestPoint6 (line 2) 
    In solutionTest (line 13)]

If the name is changed from mldivide() to mymldivide() in the uploaded file (Files Referenced) and the submitted function calls mymldivide, then it works fine (error message "Function not allowed" appears).

[Jan]

@Ales Janka: What a pity. Bruno's comment shows, that even excluding \ will not solve the problem.

Does the function inmem work in the grader to check, if the mldivide function has been used?

[John D'Errico]

I'd agree with Jan and Bruno. Overloading the mldivide function, first ensuring that your version is on top of the search path will work. You would also need to overload mrdivide too though. Just create new functions on top of the search path that cause an error return when used. So what if a warning arises?

Can the students get around such a kludge? Yes, probably. I could probably come up with multiple hacks. And, worse, if I wanted to do so, there are lots of ways to solve a backslash problem that do not use backslash. Lets see...

lsqr, lscov, regress, inv, pinv, lsqlin, ...

You don't say if the problem is square or not, but all of those functions will work in any case.

The point is, if you want to stop an enterprising student who knows what they are doing and who wants to subvert the system, you won't do so easily if you want it to be a totally automatic solution.

I suppose you could also computationally parse through the code, searching for calls to the possible outlawed solutions. Or you could check to see if ANY of the possible alternatives were woken up. I might need to extend my list above.

Instead, simple inspection seems a good idea. Just glance through the code. Or even better, tell the students they are not allowed to use specific codes to solve the problem. Then check a few random solutions by eye.

Trust works, especially if some inspection is applied to validate that students were worthy of your trust. I'd bet that you even should know which students need to have their work checked. If you have no idea which students cannot be trusted, then it might be you need to know your students better.

Answer 1

[Cris LaPierre]

There is no built-in way to prevent student solutions from using "\".

[Ales Janka]

In the meantime, I found I can open the student solution file as a text file, search through it for forbidden keywords (like the "\") and flag them by the assert function.

Unfortunately, my crude implementation does not distinguish between code and comment, in the student's functions...

Many thanks for your time, anyway!

Best,

Ales